Newtons Laws: Identifying Forces on objects One of the key concepts in Mechanics is identifying what forces are acting and drawing a Force diagram, also known as a Free Body Diagram

Identifying and Drawing forces Start by drawing the object. You can either sketch the object or draw a dot to represent it. Start by drawing the object. You can either sketch the object or draw a dot to represent it. For each separate force draw an arrow pointing away from the object in the direction that the force is acting. For example, typically a gravitational force is acting straight down (unless the object is floating in space somewhere) For each separate force draw an arrow pointing away from the object in the direction that the force is acting. For example, typically a gravitational force is acting straight down (unless the object is floating in space somewhere)

Force Diagrams (Free Body Diagrams) For each force make the length of the arrow approximate the size of the force. For each force make the length of the arrow approximate the size of the force. If 2 or more forces are acting in the same direction, indicate 2 different arrow to indicate they are different forces. If 2 or more forces are acting in the same direction, indicate 2 different arrow to indicate they are different forces. Identify each arrow: you can write F g for force of gravity and F f for force of friction, etc. Identify each arrow: you can write F g for force of gravity and F f for force of friction, etc.

Force Diagrams and Newtons Laws So, what good are these diagrams? So, what good are these diagrams? According to Newtons 2 nd law, accel = Force net / mass According to Newtons 2 nd law, accel = Force net / mass We need to determine what the Force net is for the object. We need to determine what the Force net is for the object. Like we did in studying motion, the horizontal and vertical forces are always treated separately or independently. Like we did in studying motion, the horizontal and vertical forces are always treated separately or independently.

Forces and Newtons laws If an object is not accelerating (looking at either vertical or horizontal direction) then the net Forces in that direction must add up to zero If an object is not accelerating (looking at either vertical or horizontal direction) then the net Forces in that direction must add up to zero Even if an object is moving at constant speed the net forces must be zero in the direction of motion. Even if an object is moving at constant speed the net forces must be zero in the direction of motion.